text: Carbon principally enters the mantle in the form of carbonate-rich sediments on tectonic plates of ocean crust, which pull the carbon into the mantle upon undergoing subduction. Not much is known about carbon circulation in the mantle, especially in the deep Earth, but many studies have attempted to augment our understanding of the elements movement and forms within said region. For instance, a 2011 study demonstrated that carbon cycling extends all the way to the lower mantle. The study analysed rare, super-deep diamonds at a site in Juina, Brazil, determining that the bulk composition of some of the diamonds inclusions matched the expected result of basalt melting and crystallisation under lower mantle temperatures and pressures. Thus, the investigations findings indicate that pieces of basaltic oceanic lithosphere act as the principal transport mechanism for carbon to Earths deep interior. These subducted carbonates can interact with lower mantle silicates and metals, eventually forming super-deep diamonds like the one found.
  Carbonates descending to the lower mantle form other compounds besides diamonds. In 2011, carbonates were subjected to an environment similar to that of 1800 km deep into the Earth, well within the lower mantle. Doing so resulted in the formations of magnesite, siderite, and numerous varieties of graphite. Other experiments—as well as petrologic observations—support this claim, finding that magnesite is actually the most stable carbonate phase in the majority of the mantle. This is largely a result of its higher melting temperature. Consequently, scientists have concluded that carbonates undergo reduction as they descend into the mantle before being stabilised at depth by low oxygen fugacity environments. Magnesium, iron, and other metallic compounds act as buffers throughout the process. The presence of reduced, elemental forms of carbon like graphite would indicate that carbon compounds are reduced as they descend into the mantle. 
  Nonetheless, polymorphism alters carbonate compounds stability at different depths within the Earth. To illustrate, laboratory simulations and density functional theory calculations suggest that tetrahedrally-coordinated carbonates are most stable at depths approaching the core–mantle boundary. A 2015 study indicates that the lower mantles high pressures cause carbon bonds to transition from sp to sp hybridised orbitals, resulting in carbon tetrahedrally bonding to oxygen. CO trigonal groups cannot form polymerisable networks, while tetrahedral CO can, signifying an increase in carbons coordination number, and therefore drastic changes in carbonate compounds properties in the lower mantle. As an example, preliminary theoretical studies suggest that high pressures cause carbonate melt viscosity to increase; the melts' lower mobility as a result of the property changes described is evidence for large deposits of carbon deep into the mantle.
  Accordingly, carbon can remain in the lower mantle for long periods of time, but large concentrations of carbon frequently find their way back to the lithosphere. This process, called carbon outgassing, is the result of carbonated mantle undergoing decompression melting, as well as mantle plumes carrying carbon compounds up towards the crust. Carbon is oxidised upon its ascent towards volcanic hotspots, where it is then released as CO. This occurs so that the carbon atom matches the oxidation state of the basalts erupting in such areas.